Likewise I didn't know Korpiklaani were pagan, I thought you'd call them Folk metal. Only heard them once and that was when I saw them at Graspop 2008. Wasn't a huge fan of the music but they were alot of fun live.

Pagan lyrical themes/band culture are the only thing that ties Pagan metal bands together. Otherwise they can already be described with other genre. Viking metal is a little different since it has a distinctive epic sound, mixed with black metal (i.e. defined by Bathory).

Personally I prefer my power metal to be power metal, and folk metal to be mixed heavily with black metal. i.e., Huldrefolk, Wyrd, Moonsorrow, Drudkh - and most of all, Falkenbach.

I could be totally wrong since I've only been listening to this stuff for a short time, so anyone please feel free to correct me. In my opinion it's a small difference, actually more about the lyrics than the music. Pagan metal is about pre-Christian traditions of some culture or region, but is closely related to folk metal which combines heavy metal with traditional folk music. Folk metal lyrics are about paganism, history, nature, mythology, fantasy, etc.

I'd put it this way: "Folk Metal" is a musical style. "Pagan Metal" is a collection of lyrical themes. Thus, there is nothing requiring that one have anything to do with the other. But for a variety of reasons, the two categories happen to have considerable overlap.

"Folk Metal" itself is a pretty broad categorization, since it's "what type(s) of folk music?" combined with "what type(s) of Metal?" and "in what proportions?" -- the mix & match possibilities are very numerous.